Solar power system and high efficiency photovoltaic cells used therein

ABSTRACT

A solar power system including a movable platform for tracking the sun, a radiation concentrator, and a plurality of photovoltaic cell modules positioned on the platform for receiving concentrated solar radiation. The module includes a heat dissipation housing which supports a silicon cell across an open end of the housing. A heat transfer block physically engages the silicon cell and a metallic sponge and wick is attached to the heat transfer block and depends therefrom into the housing. The housing is partially filled with liquid to facilitate heat removal. The silicon cells are processed by preferential etching to form V grooves which define a plurality of diode elements having generally trapezoidal cross-sections. The elements may be serially interconnected by metallization on the V groove surfaces. The physical configurations of the elements and the use of antireflective coatings on surfaces of the elements result in high efficiency cells.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

This invention relates generally to electric power systems, and moreparticularly the invention relates to power systems utilizing solarradiation to energize photovoltaic cells and to semiconductor PNjunction devices used in photovoltaic cells.

2. Prior Art:

Solar radiation is recognized as a potential source for large amounts ofenergy, if properly harnessed. Historically, various heat transfermechanisms have been devised for converting solar radiation into heatenergy. In more recent years considerable effort has been directed tothe conversion of solar energy to electrical energy through use of solaror photovoltaic cells. Such devices are employed in space applications,for example.

The photovoltaic cell comprises single crystalline silicon material inwhich a PN junction is formed by the selective introduction of elementaldopants into the semiconductor body. Doping techniques such as diffusionand ion implantation are well known in semiconductor processingtechnology.

In operation of the photovoltaic cell, a potential difference exists atthe PN junction of the semiconductor cell due to the diffusion ofelectrical carriers, holes and electrons, across the PN junction whichare then captured by majority carriers of the new region. By exposingthe semiconductor cell to solar radiation, incident radiation isabsorbed within the semiconductor body and will create electron-holepairs or carriers which can be separated by the PN junction and madeavailable to energize an external circuit. Only radiation or photonshaving an energy level of approximately 1.12 electron volts or highercan create an electron-hole pair in silicon. Such photons have awavelength of 1.11 microns or shorter. Photons of greater wavelengthhaving lesser energy may be absorbed by the cell as heat, and the excessenergy of the shorter wavelength photons will be wasted as heat, also.Due to only a percentage of solar radiation (approximately 45% insilicon) being available for energy conversion and since the maximumpower of a silicon photovoltaic cell is delivered at about one-half voltrather than 1.12 volts, maximum energy conversion without concentrationof radiation is about 22%. However, in practice, other losses reducethis to about 10% in conventional photovoltaic cells.

The most widely employed of conventional photovoltaic cells is theplanar junction device introduced by Chapin and Fuller at the BellTelephone Laboratories in the mid-1950's. In such devices a PN junctionis formed near a radiation receiving surface of a semiconductor body.Metallic electrode fingers are placed on the surface of thesemiconductor body to form a current collection grid for the cell. Dueto shadow loss from the collection grid on the radiation receivingsurface and due to series resistance losses in the cells, the intrinsicefficiency of planar junction devices is substantially less than 20%.

The interdigitated back contact cell developed by Lammert and Schwartzof Purdue University eliminates the shadow loss by providing alternatingP & N type regions on the back surface of a semiconductor body with theP regions connected in parallel and the N regions connected in parallel.The interdigitated contacts reduce series resistance losses, also. Inthis cell a silicon oxide layer is provided on the top surface tominimize the hole-electron recombination at the surface. Limitations ofthe cell include not only the more complex semiconductor processing infabricating the device but also difficulty in optically matching thecell to the outside world.

Another prior art device is the vertical multi-junction cell proposed bySater of NASA. This device is fabricated from a stack of semiconductorwafers having alternating N and P type conductivity and in which a thinaluminum layer is provided between wafers for adhesion purposes. Thestack is then sliced to provide a cell with alternating P & Nsemiconductor regions, each separated by a thin aluminum layer. Thisdevice is characterized by a high surface recombination of holes andelectrons because of the difficulty of surface passivation due to thepresence of the aluminum material. Further, uniformity of thesemiconductor regions is difficult to maintain due to the plurality ofsemiconductor wafers used in forming the stack.

Several embodiments of a monolithic photovoltaic semiconductor deviceare disclosed in U.S. Pat. No. 3,994,012 to Warner which utilize aplurality of series connected PN junctions in the monolithic body. Onemajor limitation of the Warner devices is the involved and complexprocessing including several high temperature steps in fabricating thedevice. Additionally, the necessary electrical isolation betweenadjacent PN junction elements is difficult to achieve without the use ofadditional "bucking" or isolating PN junctions. Further, the Warnerdevice utilizes a metal layer on the illuminated surface therebycreating a shadow loss in device operation.

SUMMARY OF THE INVENTION

An object of the present invention is an improved solar electric powersystem.

Another object of the invention is a solar electric power system whichutilizes a photovoltaic cell module with improved heat dissipationcharacteristics.

Still another object of the invention is a photovoltaic cell withimproved energy conversion efficiency.

Another object of the invention is a photovoltaic cell comprisingsemiconductor elements which are easily fabricated.

Yet another object of the invention is a simple process for fabricatinga silicon cell.

Features of the invention include a photovoltaic cell which utilizessemiconductor elements each having a generally trapezoidal crosssection.

Another feature of the invention is a semiconductor cell having aplurality of V grooved surfaces.

Another feature of the invention is a heat transfer mechanism in which aheat transfer block comprising semiconductor material is provided with asurface configuration which mates with the surface configuration of aphotovoltaic cell for increased conduction of heat from the cell.

Briefly, the solar electric power system in accordance with theinvention includes a support means with radiation concentrating meanscooperatively mounted with the support means for receiving andconcentrating solar radiation. A photovoltaic cell module is mounted onthe support means to receive the concentrated solar radiation. Themodule includes a heat conductive housing having an open end, asemiconductor cell mounted across the open end between a transparentcover and a heat transfer block. A metallic sponge and wick means isattached to the heat transfer block and depends therefrom into thehousing. The housing may be partially filled with a liquid to facilitatethe heat transfer from the heat transfer block to the housing and thenceto the outside ambient.

The photovoltaic cell comprises a plurality of single crystallinesemiconductor elements each having a generally trapezodial crosssection. The semiconductor elements are mounted to suitable supportmeans for exposure to solar radiation from the concentrator. Because ofthe configuration of the semiconductor elements, each element can havesmaller physical size than conventional planar junction devices whileretaining efficient utilization of impinging photons. Further, theelements may be serially interconnected by low resistance means.Additionally, the elements are easily fabricated from a semiconductorwafer using conventional semiconductor processing techniques.

The invention and objects and features thereof will be more fullyunderstood from the following detailed description and appended claimswhen taken with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one embodiment of a solar electric powersystem in accordance with the present invention.

FIG. 2 is a side view partially in section of a photovoltaic cell modulein accordance with the present invention.

FIG. 3 is a perspective view in section of one embodiment of aphotovoltaic cell in accordance with the present invention.

FIGS. 4-6 are side views in section illustrating the steps infabricating the photovoltaic cell shown in FIG. 3.

FIGS. 7-12 are each perspective views in section of other embodiments ofa photovoltaic cell in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a perspective view of a solar electric power system inaccordance with the present invention. A support platform 10 is mountedon a support pedestal 12 which houses motor drive means which is coupledthrough suitable gear mechanisms to rotate the support platformvertically and horizontally as indicated, for solar tracking purposes.Mounted above platform 10 by suitable means such as support struts 14 isa radiation concentrator 16 such as a square acrylic Fresnel lens. Itwill be appreciated that other concentrator means may be employed suchas, for example, mirror arrangements and dye concentrators. Mounted onplatform 10 is photovoltaic cell module (not shown) above which may bepositioned a secondary concentrator 18 such as a compound parabolic lensor cone concentrator. Additionally, a selective absorber can be includedbelow the secondary concentrator to filter out the longer wavelengthradiation which has insufficient energy to form electron-hole pairs inthe photovoltaic cell. One such selective absorber is a thin layer (e.g.one centimeter deep) of water positioned between parallel glass plates.Such a filter can reduce approximately 20% of incoming radiation andthus prevent heating of the photovoltaic cell by radiation which hasinsufficient energy to create electron-hole pairs.

FIG. 2 is a side plan view partially in section of the photovoltaic cellmodule which is mounted to support platform 10 beneath the secondaryconcentrator 18 shown in FIG. 1. The module includes a heat conductivehousing 20 made of suitable material such as aluminum. Housing 20 can beprovided with a plurality fo fins 22 to facilitate the removal of heatby convection. The upper end of housing 20 is open and receives a solarcell 24 which is supportably mounted to a glass plate 26 and electricalleads 28 and 30. Electrical leads 28 and 30 provide for electricalconduction of electricity from the solar cell to an output bus bar 32.

As shown in the section view, lead 28 is fastened by screw means to busbar 32 which is formed in an insulated portion of the support platform10. Insulative gasket 34 provides electrical insulation between lead 28and housing 20. A similar bus bar arrangement is provided for lead 30(not shown).

In accordance with one feature of the invention, the solar cell 24 has agenerally trapezoidal cross section which, as will be described furtherhereinbelow, may preferably be formed by etching grooves in asemiconductor body. A heat transfer block 36 abuts solar cell 24 tofacilitate removal of heat from the cell. Advantageously, the heattransfer block may comprise a body of silicon material which has beenprocessed in a manner similar to the surface of the semiconductor waferfrom which solar cell 24 is fabricated whereby the cell and the heattransfer block have mating generally trapezoidal cross sectionedsurfaces, as shown. Alternatively, as will be described hereinbelow, theheat transfer block may comprise other material which is affixed to thesolar cell by suitable means such as epoxy for the removal for heat.

Solderably attached to the lower surface of heat transfer block 36 is ametallic sponge and wick 38 which forms a resilient support between theheat transfer block 36 and housing 20. The solder joint provides adurable bond and the sponge and wick permit good heat transfer throughthermal cycling of materials which might have different coefficients ofthermal expansion, such as silicon heat transfer block and the metallichousing. Suitable materials are available from Foametal Inc.,Willoughby, Ohio, for example.

Housing 20 will be partially filled with a liquid such as water whichflows by capillary action through wick 38 to the heat transfer block 36.The liquid then vaporizes from the heat from block 36, and the vaporcondenses elsewhere within housing 20 thus providing for efficienttransfer of heat to the housing walls. Heat is then removed from housing20 through convection to the outside atmosphere, which is facilitated bymeans of fins 22.

Electrical contact is made to the photovoltaic cell module by means ofthe same two screws which fasten the cell module to the concentratorbody. A pair of bus bars is present in the concentrator body; flexibleleads from the cell are brought into contact with these two bars whenthe module is installed. Additionally, the cell can be sealed from theenvironment by a sealant applied around the region where the electricalleads 28 and 30 are soldered to the solar cell.

Since the efficiency of a photovoltaic cell decreases with increasingoperating temperature and since the photovoltaic cell in accordance withthe present invention is designed to operate with radiationconcentration of 10¹ -10⁵ times, effective heat dissipation is ofparamount importance. The module design herein described is particularlyefficient for heat removal and significantly contributes to the higherefficiency of the photovoltaic cell in accordance with the presentinvention.

Referring now to FIG. 3, one embodment of a photovoltaic cell ispartially illustrated in section in a perspective view. The full lengthof the cell is not shown, however the cell can be as long as thesemiconductor wafer from which it is fabricated. Two elements 50 and 52of a photovoltaic cell are shown. However, it will be appreciated that acell will have many elements serially connected. Each element comprisesa single crystalline semiconductor body with a generally trapezodialcross section. In this embodiment the bases of the elements are attachedto a glass plate 54 through which solar radiation may be received by theelements. One glass which has good optical transparency and a closelymatched coefficient of thermal expansion with silicon is 7070 glass fromthe Corning Glass Company. As will be further described, the glassprovides a supporting substrate for the silicon cells and protects theelements from contamination. Between the glass plate 54 and the elements50, 52 is a thin layer of an anti-reflective coating 56, tantalum oxidefor example, which reduces optical reflection loss due to thedifferences in indices of refraction of the glass and silicon. Siliconoxide layer 58 may be thermally grown in the fabrication of the siliconelements and provides for low surface recombination of holes andelectrons.

In one embodiment, the silicon element have a bulk dopant concentrationof 10¹³ -10¹⁴ atoms per cubic centimeter and more heavily doped N+ andP+ regions are formed in opposing sides of the elements by introducingan N-type dopant such as phosphorous or arsenic into surface regions 60and 62 and a P type dopant such as boron into surface regions 64 and 66.As will be described further below, the N+ regions and the P+ regionsare advantageously formed by an ion implantation process. The bulkconductivity of the elements can be either P type or N type. Adjacentelements are serially connected by depositing a metal conductive layer68 in the V groove between adjacent elements in contact with the P+region 64 of element 50 and the N+ region 62 of element 52. Thus, a verylow resistance interconnection can be provided for the solar cell.

The surface 70 and 72 of the elements, which are opposite the surfacesof elements 50 and 52 abutting glass plate 54, are provided with siliconoxide layers 74 and 76, respectively, to which metallic layers 78 and 80are deposited. The metallic layers 78 and 80 provide for reflection ofradiant energy back into the cell.

In the described cell, photons which have not been absorbed to producean electron-hole pair, can be reflected internally from themetal-covered regions on the back of the cell and from the metallizedjunction regions. Over 90% reflection at wavelengths of 600 nm or longercan be obtained. Further, most photons reflected upward from the metalcovered regions on the backside of the cell and from the metallizedjunction regions will be reflected back into the cell by the process oftotal internal reflection from the silicon surface adjacent to siliconoxide layer 58. Consequently, the effective optical thickness of thecell, and hence, its collection efficiency of solar photons can begreater than that of a conventional planar cell having a thickness equalto the height of the trapezoidal elements.

The enhanced photon absorption capability of the cell allows it to bemade physically thin while retaining almost complete absorption of solarphotons having greater than the bandgap energy of silicon (1.12 eV at27° C.). Because of this and because silicon material is removed fromthe backside of the cell, the ratio of the illuminated area of the cellto its volume can be much larger than that in a conventional planar cellhaving an identical collection efficiency of solar photons. The largerratio of the cell illuminated area to the cell volume produces a highercarrier concentration in the cell than in the planar cell. This givesthe cell a higher open-circuit voltage and operating voltage than thatof the planar cell.

In accordance with another aspect of the invention, the photovoltaiccell is readily fabricated using conventional semiconductor processingtechniques whereby a single photoresist masking step is employed. FIGS.4-6 are sided views in section of the silicon cell of FIG. 3 duringfabrication and illustrate the simple fabrication process. Like elementsof FIGS. 4-6 and FIG. 3 have the same reference numerals.

Initially, a P-type silicon wafer having a surface crystal orientationof 1-0-0 and a thickness of 50 microns and a resistivity of 13ohm-centimeter is thermally oxidized in steam at a temperature ofapproximately 1000° C. to form silicon oxide layer 58 and a similarthermal oxide layer on the opposing surface of the silicon wafer. Theoxide thickness is preferably about 0.5 micron. To obtain a low surfacerecombination of hole and electrons, a low surface-state density of thesilicon oxide is required. The low surface state values can be obtainedby following the thermal oxidation of the silicon wafer with a nitrogenannealing step. The silicon oxide layer on the surface of the siliconwafer opposing layer 58 is protected with photoresist and silicon oxidelayer 58 is thinned to a thickness of about 250 angstroms by etching indilute hydrofluoric acid.

Thereafter, the anti-reflective coating 56 is formed over the siliconoxide layer 58 by the reactive sputtering of tantalum which deposits astantalum oxide on the surface. Alternatively, tantalum may be vapordeposited on the surface and subsequently oxidized to obtain tantalumoxide. Other known anti-reflective materials such as titanium oxide maybe used, also. Preferably, the anti-reflective coating has an opticalthickness of a quarter the wavelength of radiation in theanti-reflective coating at 5500 angstroms, or approximately 500angstroms thickness.

The oxidized silicon wafer is attached to the glass plate 54 by means ofa conventional field assisted bonding process (FAB). The silicon wafer82 and glass plate 54 are stacked with the silicon oxide layer 58 andanti-reflection coating 56 therebetween, and the stacked structure issandwiched between a polished silicon wafer and another glass plate withlike materials touching each other. This composite structure is thenplaced between graphite blocks to which electrical connections are made,and the stack is then heated to 500° C. in a quartz vacuum furnace. Withthe negative polarity on the glass side, 1,000 volts DC is applied tothe stack for approximately ten minutes. This procedure results in theglass plate bonding to the silicon wafer at the anti-reflective coatinginterface.

Next, the single photoresist masking step employed in the process isused to define the oxide strips 74 and 76. Standard photoresist maskingand etch techniques are employed to define the strips. The dimensions ofthe silicon oxide strips and spacing thereof are important in thesubsequent processing of the silicon wafer when forming V grooves whichdefine the individual elements of the silicon cell. For a silicon waferof 50 microns thickness, the strip width is 17.4 microns and stripspacing is 65 microns. A preferential etchant can be applied to asilicon wafer having 1-0-0 crystalline orientation which etches into thewafer at an angle of 54.7° from the surface of the wafer with the normalto the V grooved surfaces aligned parallel to the 1-1-1 crystalline axisof the silicon wafer. Thus, by appropriate dimensioning of the siliconoxide strips, which act as a mask against the preferential etchant, Vgrooves are etched through the silicon wafer to the underlying siliconoxide layer 58 to obtain a desired generally trapezoidal cross sectionfor the silicon element. As noted in FIG. 5, the preferential etchingprocess results in a slight undercut of the silicon beneath the oxidestrips 74 and 76 which is advantageously utilized in the forming of theelectrical conductive pattern as will be described further hereinbelow.

Referring to the dimensions illustrated in FIG. 6, the relationship ofthe strip width, W, to strip spacing, S, for a wafer height, H, is givenas follows:

    S=(2H/tan θ)+D-2U

    W=R+2U

    U=(3/50) H/Sin θ

where

D is the spacing of elements at the silicon oxide interface at theilluminated surface

U is the undercut of silicon beneath one edge of each strip

θ is the angle of the etched surface

R is the width of the silicon beneath the oxide strip

Preferential etchants and etching processes for silicon are well-knownin the semiconductor art. In one embodiment a mixture of 120 millilitersof ethylenediamine, 22 grams of pyrocatechol, and 60 milliliters ofwater is used in a reflux condensor-fitted pyrex container to performthe V groove etching. The etchant temperature is maintained at 104°-110°C. during the approximately 50 minutes necessary to etch through a 60micron thick silicon wafer. After etching, the glass plate and siliconelements are cleaned in hot methyl alcohol and rinsed in deionizedwater.

After the V grooves are formed in the silicon wafer to define theplurality of cell elements, the structure is placed in an ionimplantation chamber and rotated 33° from the normal in one directionand N type dopant ions are directed towards the exposed silicon surfaceto form the N+ regions 60 and 62. Thereafter, the wafer is rotated 33°from normal in the opposite direction and exposed to P type ions toimplant the P+ regions 64 and 66. The rotational alignment of the waferprevents the particular dopants from implanting in the regions ofdesired opposite conductivity, and the oxide strips 74 and 76 provide amask preventing the ions from entering the underlying silicon surface.Dual energy implants are preferably used to form the junction at adesired depth and with high surface concentration. In accordance withconventional ion implantation techniques, phosphorous ions are implantedat an energy of 150 keV for the N+ regions, and boron ions are implantedat an energy of 60 keV for the P+ regions, to give an 0.4 micronjunction depth in each region. For the low energy implant 40 keV is usedfor phosphorous and 20 keV is used for boron to provide a high surfaceconcentration of the dopants. Doses of 5×10¹⁵ atoms per squarecentimeter and 10¹⁴ atoms per square centimeter are used for the highand low energy implants, respectively. The ion implantation is followedby an annealing step to minimize residual crystal lattice disorder andto obtain better quality junctions.

As shown in FIG. 6, the cell is completed by depositing metalinterconnection 68 and reflective coatings 78 and 80 on the siliconelements. The metallization may comprise aluminum which is evaporated toa thickness of approximately 1 micron. Shorting around the ends of thediodes is prevented by using a metal shadow mask during evaporationwhich covers the end portions of the elements. Importantly, because theoxide strips 74 and 76 are undercut several microns during thepreferential etching step, the metallization is discontinuous over theedges of these strips. Thus, while the aluminum metallizationinterconnects adjacent diode elements in series, the N+ and P+ regionsof individual elements are not shorted. Consequently, no photomaskingand etching step is required to remove unwanted metal.

In one embodiment, a two inch diameter silicon wafer was processed. Fourcells, one in each quadrant of the wafer, were made using the processesoutlined above. Each cell measure 0.49 centimeter on the side, 0.24centimeter² in total area, and contained 43 individual diode elements.The four cells were separated by conventional scribe and breaktechniques. Flexible electrical leads were then connected, and the cellwas mounted in a module.

FIG. 7 is a perspective view of another embodiment of a silicon cellshown partially in cross section which is very similar to the embodimentillustrated in FIG. 3. Like elements have the same reference numerals.In this embodiment the N+ regions 60 and 62 are extended across thebonded surfaces of elements 50 and 52, respectively, as shown at 60' and62'. Alternatively, the P+ regions 64 and 66 could be so extended. Ineither embodiment, the N+ region and the P+ region of each element arenot in contact. Thus, each element is provided with a PN junction,defined by the N+ region and the P type bulk silicon material, whichprovides for a low surface recombination of holes and electrons.Accordingly, there is no need for a silicon oxide layer at the radiationreceiving surface of the cell. This embodiment is fabricated in aprocess similar to that described with reference to FIGS. 4-6 exceptoxide layer 58 is removed and the regions 60' and 62' are formed by ionimplantation of an N type dopant such as phosphorous or arsenic into thesurface prior to attachment of the silicon wafer to the plate 54 andV-groove etching of the silicon wafer.

FIG. 8 is another embodiment of a photovoltaic cell in accordance withthe invention which is similar to the embodiment of FIG. 3, and againlike elements have the same reference numerals. However, in thisembodiment the N+ regions 60" and 62" and the P+ regions 64" and 66" areformed in the surface of each element 50 and 52, respectively, which isopposite to the surface bonded to glass plate 54. Silicon oxide isprovided under the metal interconnect 68. The cell is particularlyeffective at trapping light, thus allowing the cell to be made quitethin and attain a high collection efficiency and high open circuitvoltage. However, the process in manufacturing the device is somewhatmore complicated as additional masking steps are required in definingthe regions 60", 62", 64" and 66". In fabricating these regionsphotoresist masking techniques must be employed to define ionimplantation or diffusion windows for the two N+ regions and for the P+regions and the N+ and P+ regions must be formed prior to the etchingstep. Further masking and etching steps are required in providingcontact openings and in defining the metallic interconnection pattern tothe N+ and P+ regions. Thus, a total of five masking steps are requiredin the fabrication.

FIGS. 9-12 are perspective views partially in section of portions ofother embodiments of photovoltaic cells in accordance with the inventionwherein the silicon elements 100 and 102 are supportably mounted on anunderlying substrate 104. Referring to FIG. 9, the bottom surfaces ofthe elements are provided with a silicon oxide coating 106 on which amaterial such as polysilicon is deposited by the chemical decompositionof tricholorosilane. Other substrate material can be provided as asubstrate using the FAB process described above. V grooves are etched inthe silicon material using the preferential etch process described abovewith reference to FIGS. 4-6 with N+ region 108 and P+ region 110 formedby ion implantation, as above. Silicon oxide strips 107 are provided onthe top surface which selectively mask the silicon wafer for subsequentpreferential etching. Following the ion implantation step, a thin layerof metal 112 (e.g. 100 angstrom thickness) is deposited on oxide layer107 and on the V groove surfaces to interconnect adjacent elements.Thereafter, an antireflective coating 114, such as tantalum oxide, isdeposited on the metal interconnect layer 112 and on metal layer 113 tocomplete the cell. In an alternative embodiment, the antireflectivecoating and thin metal can be replaced by a layer of indium-tin oxide.

FIG. 10 is an embodiment similar to the configuration of FIG. 9 and inwhich the N+ and P+ regions are formed in the surface of each elementopposite to the surface abutting the substrate. In this respect, theembodiment is similar to the embodiment described with reference to FIG.8. As in the embodiment of FIG. 8 the N+ and P+ regions 120 and 122 areformed in the silicon wafer by ion implantation or by diffusion ofdopants prior to the preferential etch of the silicon wafer. However,unlike the embodiment of FIG. 8, impinging light from the concentratorfirst strikes the surface in which regions 120 and 122 are formed, andto improve the receptivity of the silicon to the impinging photons thesurface 123 between regions 120 and 122 can be given a textured surfaceby applying the preferential etchant thereto for a limited period oftime. By so doing, a plurality of pyramidal shapes are formed on thesurface which are defined by the 1-1-1 crystal orientation planes. Themetal interconnection 112 is applied over silicon oxide in the Vgrooves.

In the embodiment illustrated in FIG. 11 the N+ region 124 and the P+region 126 of each element is defined in a surface of the silicon waferby ion implantation or by diffusion techniques prior to the applicationof substrate 104 thereto. Importantly, it should be noted that the P+region 126 of one element abuts the N+ region 124 of an adjacent elementthus creating a tunnelling junction which provides serial electricalconnection between adjacent elements. The V grooved surfaces of theelements are provided with a layer of silicon oxide and a layer ofanti-reflective coating material shown generally at 128. Since theelements are serially interconnected by the tunnelling junction, ametallic interconnection pattern is not required on the element.

The embodiment illustrated in FIG. 12 is similar to the embodiment ofFIG. 11. However, N+ and P+ regions of adjacent elements do not abut asin FIG. 11. In this embodiment a metallic interconnection pattern 130 isdeposited over silicon oxide on the silicon wafer surface tointerconnect the N+ region 124 of one element to the P+ region 126 ofthe adjacent element prior to the formation of substrate 104 thereon.Again, layers of silicon oxide and an antireflective coating showngenerally at 128 are provided over the V grooved surfaces of theelements as in FIG. 11.

Solar cells in accordance with the present invention realize conversionefficiencies of 20% and higher. Because of the V groove configurationthe elements can have a small geometry while still achieving a high rateof photon capture. The silicon cells are easily fabricated usingconventional semiconductor processing techniques and in at least oneembodiment a single mask fabrication procedure is employed. The very lowseries resistance of the elements permits efficient operation of thesolar cell in sunlight concentrated 1,000 times or more. Further, themodule provides efficient transfer of heat from the solar cell thuspermitting an improved solar electric power system using high solarconcentration.

As used herein, the term silicon oxide includes silicon dioxide, asconventionally used in semiconductor processing. Diffusion of dopantsmay include dopant ion implantation. Further, while the preferredembodiment utilizes silicon as the semiconductor material, othersemiconductor material might be used in practicing the invention. Thus,while the invention has been described with reference to specificembodiments, the description is for illustrative purposes only and isnot to be construed as limiting the invention. Various modifications,changes, and applications may occur to those skilled in the art withoutdeparting from the true spirit and scope of the invention as defined bythe appended claims.

What is claimed is:
 1. A solar electric power system comprising supportmeans,concentrator means cooperatively mounted with said support meansfor receiving and concentrating solar radiation, a photovoltaic cellmodule mounted on said support means to receive said concentrated solarradiation, said module including a heat conductive housing having anopen end, a photovoltaic semiconductor cell mounted across said open endin abutment with a heat transfer block, said semiconductor cellincluding a V grooved surface and said heat transfer block having amating V grooved surface for receiving said semiconductor cell, metallicsponge and wick means attached to said heat transfer block and dependingtherefrom and into said housing, and liquid means within said housingfor facilitating heat transfer.
 2. A solar electric power system asdefined by claim 1 wherein said heat transfer block comprises siliconmaterial.
 3. A solar electric power system as defined by claim 1 whereinsaid photovoltaic semiconductor cell comprises a plurality of singlecrystal semiconductor elements of one conductivity type with PNjunctions defined therein, each element having a generally trapezoidalcross section, a top surface of each of said elements being bonded to aglass cover with said V shaped grooves being inverted.
 4. A solarelectric power system as defined by claim 3 wherein said semiconductorelements are bonded to said glass cover by means of an anti-reflectivematerial and a layer of silicon oxide.
 5. A solar electric power systemas defined by claim 3 wherein each of said semiconductor elements has abulk conductivity of a first type, a first region of doped semiconductormaterial of said first conductivity type in a first grooved surfaceregion of said element, and a second region of doped semiconductormaterial of opposite conductivity type in a second grooved surfaceregion of said element.
 6. A solar electric power system as defined byclaim 1 wherein said concentrator means includes an optic lens.
 7. Asolar electric power system as defined by claim 6 wherein saidsupporting means is movable about at least one axis.
 8. For use in asolar electric power system, a photovoltaic cell module comprising aheat conductive housing having an open end, a photovoltaicsemiconductive cell mounted across said open end in abutment with a heattransfer block, said semiconductor cell having a V grooved surface andsaid heat transfer block having a mating V grooved surface for receivingsaid semiconductor cell, metallic sponge and wick means attached to saidheat transfer block and depending herefrom and into said housing, andliquid means within said housing for facilitating heat transfer.
 9. Aphotovoltaic cell module as defined by claim 8 wherein said blockcomprises silicon material.
 10. A photovoltaic cell module as defined byclaim 8 wherein said photovoltaic semiconductor cell comprises aplurality of single crystal semiconductor elements of one conductivitytype, each element having a generally trapezoidal cross section, a glassplate, means attaching one side of each element to said glass platewhereby each element can receive solar radiation through said glassplate, a first region of doped semiconductor material of said oneconductivity type formed in a first surface region of said element, anda second region of doped semiconductor material of opposite conductivitytype formed in a second surface region of said element, and meanselectrically interconnecting said semiconductor elements.
 11. Aphotovoltaic cell module as defined by claim 10 wherein saidsemiconductor elements are bonded to said glass plate by means of anantireflective material and a layer of silicon oxide.
 12. A photovoltaiccell module as defined by claim 10 wherein each of said elements has abulk conductivity of a first type, a first region of dopedsemiconductive material of said first conductivity type in a firstgrooved surface region, a second region of doped semiconductive materialof opposite conductivity type in a second grooved surface region.
 13. Aphotovoltaic cell having a grooved surface adapted to mate with agrooved surface of a heat transfer block comprising a plurality ofsingle crystal semiconductor elements of one conductivity type, eachelement having a generally trapezoidal cross section which defines saidgrooved surface, a supporting glass plate, means attaching the base ofeach said trapezoidal element to said glass plate whereby each elementcan receive solar radiation through said glass plate, a layer ofreflective material on a second side of each element which is oppositeto said one side attached to said glass plate, a first region of dopedsemiconductive material of said one conductivity type formed in a firstsurface region of each said element, and a second region of dopedsemiconductive material of opposite conductivity type formed in a secondsurface region of each said element, and means electricallyinterconnecting said semiconductor elements.
 14. A photovoltaic cell asdefined in claim 13 wherein said means for attaching said semiconductorelements to said glass plate comprises a layer of antireflectivematerial and a layer of silicon oxide.
 15. A photovoltaic cell asdefined by claim 13 wherein said elements are defined by a plurality ofgrooves formed in a semiconductor body and wherein said first surfaceregion of each cell lies along a first grooved surface of said cell andsaid second surface region of each cell lies along a second groovedsurface of said cell.
 16. A photovoltaic cell as defined by claim 15wherein said means for electrically interconnecting said elementscomprises a layer of metal formed on grooved surfaces of adjacentelements.
 17. A photovoltaic cell as defined by claim 16 wherein saidmeans for electrically interconnecting said elements comprises areflective material.
 18. A photovoltaic cell as defined by claim 15wherein said second surface region additionally lies along said surfaceof said cell which is attached to said glass plate.
 19. A photovoltaiccell as defined by claim 18 wherein said means for electricallyinterconnecting said elements comprises a layer of metal formed ongrooved surfaces of adjacent elements.
 20. A photovoltaic cell asdefined by claim 19 wherein said means for electrically interconnectingsaid elements comprises a reflective material.
 21. A photovoltaic cellas defined by claim 13 wherein said first surface region and said secondsurface region lie in a surface of each element which is opposite tosaid side of each element which is attached to said glass plate.
 22. Aphotovoltaic cell as defined by claim 13 wherein said one conductivitytype is N type and said opposite conductivity type is P type.
 23. Aphotovoltaic cell as defined by claim 22 wherein each of said elementshas a dopant concentration of approximately 10¹⁴ atoms per cubiccentimeter and said first and second surface regions have a surfacedopant concentration on the order of 10¹⁹ -10²⁰ atoms per cubiccentimeter.
 24. A photovoltaic cell as defined by claim 13 and furtherincluding a layer of optically transparent and electrically insulatingmaterial on said second side of each element with said layer ofreflective material on said layer of optically transparent andelectrically insulating material.
 25. A photovoltaic cell as defined byclaim 24 wherein said layer of optically transparent and electricallyinsulating material extends beyond the edges of said second side of eachelement.
 26. A photovoltaic cell as defined by claim 24 wherein saidlayer of optically transparent and electrically insulating materialcomprises silicon oxide.
 27. A photovoltaic cell comprising a pluralityof single crystal semiconductor elements of one conductivity type, eachelement having a generally trapezoidal cross section, a supportingsubstrate, means attaching one side of each element to said substrate, afirst region of doped semiconductor material of said one conductivitytype formed in a first surface region of each said element, a secondregion of doped semiconductor material of opposite conductivity typeformed in a second surface region of said element, means electricallyinterconnecting said semiconductor elements, said first surface regionand said second surface region lying in said surface of said elementwhich is attached to said substrate.
 28. A photovoltaic cell as definedin claim 27 wherein doped regions of adjacent elements are in physicalcontact thereby providing electrical interconnection between elements.29. A photovoltaic cell as defined by claim 27 wherein said means forelectrically interconnecting cells comprises a layer of metal.